Butter a shallow baking dish or cookie sheet generously, and set aside.

In a cast-iron pot or heavy saucepan, stir together the sugar and milk. Heat to between 234 and 240 degrees F (112 to 116 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.

Remove from the heat, and stir in the peanut butter until smooth, then stir in the vanilla. Pour onto the prepared pan, and refrigerate for about 45 minutes. Break into pieces when cooled.